The present invention relates to threaded fasteners that lock when set or tightened on a workpiece and which develop a predetermined clamp-up load while being set.
Typical fasteners consist of a nut and bolt. The nut has an internal thread that screws onto the external thread of the bolt. Wrenching surfaces on the nut and bolt accept wrenches that tightly join the fasteners and one or more workpieces together. Somewhat generally speaking, another name used for a bolt is a threaded pin, and another name for a nut is a collar.
In many applications in which fasteners are used, the fasteners must have high strength and resistance to inadvertent unscrewing which would loosen a joint. Fasteners must often bear loads not only along their longitudinal axis, but transversely across the axis. More particularly, when fasteners join together two or more sheets, the sheets are often loaded in their planes with different loads. One sheet tends to slide over the other. Fasteners passing through both sheets become loaded in shear during their resistance to this type of loading.
Axial loads arise by the clamping of fastened sheets between a head of the pin on one side of the sheets and the collar on the other side of the sheets. Fasteners quite often must respond well in applications where they are cyclically stressed under conditions that could give rise to fatigue failure. A fastener with adequate clamp-up load on it tends to resist fatigue failure better than one that is not as tightly clamped against the workpiece. On the other hand, excessive clamp-up load applies a preexisting tension on the pin which, in conjunction with loads applied by the workpiece, may unduly stress the fastener.
Thus, it is desirable to know the clamping load the fastener applies to a workpiece. Clamp-up load correlates to the resistance of a collar to further threading onto a pin. As clamp-up load force increases, the resistance to further threading increases, and the torque required to turn the collar increases. This accounts for the use of torque wrenches for setting collars. Torque wrenches are, of course, cumbersome in use.
Fasteners have, therefore, been designed for providing a predetermined clamp-up load when set. One type of fastener employs a wrenching section connected to a collar by a frangible breakneck that breaks upon application of a predetermined torque that corresponds to the desired clamp-up load. Although such fasteners find substantial use, there are known disadvantages.
Another type of fastener with a predetermined clamp-up load is described in U.S. Pat. No. 4,260,005, an eponymous fastener often referred to as an Eddie bolt because of the inventor Edgar L. Stencel. In this type of fastener, there are a plurality, typically three, of longitudinally extending lobes on the outside of the collar. The Eddie bolt collar is threaded onto the pin by a driver having a deltoid shaped cavity, the sides of which engage the lobes. When the resistance to rotation of the collar increases to a predetermined level after the collar engages the surface of the workpiece being fastened, the lobes yield in radial compression and displace collar material radially inwardly of the lobe. The deformation of the lobes occurs at a selected clamp-up load on the workpiece depending on a variety of factors such as the size and shape of the lobes, fastener size, collar material and the like.
It is an obviously desirable feature of a fastener that it does not come apart in service. Various devices have been used to keep a collar and pin together. In the Eddie bolt fastener, there are a plurality of longitudinally extending flutes in a portion of the external thread on the pin. These flutes provide void volumes interrupting the thread. When the lobes yield in radial compression, collar material radially inwardly from such lobes plastically deforms and displaces inwardly into such void volumes for locking engagement with the pin, thereby inhibiting unscrewing of the collar from the pin.
The Stencel patent illustrates a self-locking fastener with three longitudinally extending lobes on the outside of the collar, and six longitudinally extending flutes in the external thread on the pin. It was later realized that it was desirable to avoid a number of flutes which is an integral multiple of the number of lobes on a collar. In such an arrangement, there are occasional positions of the collar on the pin where the lobes are not radially aligned with a flute, and secure locking may not be obtained.
For such reasons, commercial Eddie bolts now typically have five flutes approximately 72.degree. apart around the circumference of the thread, and the corresponding collars have three lobes. Even in such an embodiment, there remains a small but finite set of rotational positions where there is poor alignment between the lobes and flutes. Thus, the resistance to unscrewing of the collar from the pin may vary from fastener to fastener. It is desirable to provide a fastener with a greater uniformity of resistance to unscrewing.